Challenges and progresses in the chemical investigation of high molecular weight species in condensed pyrolysis products of coal and biomass

The condensed products of pyrolysis embrace a wide range of compounds, ranging from relatively low molecularweight (MW) hydrocarbons ("light tar") to progressively heavier species, including large PAHs ("heavy tar"),ending up with fine carbonaceous particles (soot) of nanometric and, in some cases, even micrometric size. Whilemost part of "light tar" from either coal or biomass pyrolysis can be effectively detected by conventional gaschromatography/mass spectrometry (GC/MS), a detailed analysis of "heavy tar" and soot turns out to be rathercumbersome and more susceptible of errors, especially when the techniques, originally developed for fossil fuels-derived products, are applied to the oxygen rich biomass products. Size exclusion chromatography (SEC) coupledwith UV-Visible (UV-Vis) absorption detector is applied to both coal and biomass "heavy tar", providing evidenceof high mass components reaching MWs of thousands of u, similar to incipient soot. Carbon particles reach themicrometric scale in some coal pyrolysis products, so that "ad hoc" solvent-based separation procedures havebeen developed in order to distinguish soot from char fines. In biomass pyrolysis products, carbon rich partic-ulate remains confined to the nano-size range. The latter exhibits fluorescence emission in the green wavelengthrange, with a relatively high quantum efficiency, and can therefore be regarded as carbon dots. Recent progressshows that the molecular mass and size of the heaviest species in biomass heavy pyrolysis products can beassessed by SEC with a fluorescence detector. However, the technique still requires refinement due to the lack ofSEC calibration data valid for all tar species. Laser desorption ionization time of flight mass spectrometry (LDITOFMS) turns out to be a valid support to evaluate the molecular range of such ill-defined products.